Selective gold plating of semiconductor contacts



July 30, 1963 MOCANU 3,099,576

SELECTIVE cow PLATING OF SEMICONDUCTOR CONTACTS Filed June 24, 1960 IMMERSE OBJECT IN HOT SOLUTION OF NOOH 0r KOH WHILE OBJECT REMAINS IMMERSED PROGRESSIVELY DILUTE HYDROXIDE SOLUTION TO FORM RINSE BATH OF CLEAR HOT WATER WHILE OBJECT REMAINS IMMERSED ADD GOLD PLATING AGENT TO RINSE BATH TO FORM PLATING BATH ADJUST AND MAINTAIN TEMPERATURE OF PLATING BATH TO 80: 5C.

ADD NH OH TO PLATING BATH AS NECESSARY TO MAINTAIN pH OF 7-8 INVENTOR.

v I TIMOTHY MOCANU ATTORNEY 3,l9,576 SELECTIWE GGLD PLATHNG F SEE ll- CGNDUCTCR CUNTACT?) Timothy Mocanu, Quincy, Mass, assignor to Clevite Corporation, Cleveland, Ulric, a corporation of @hio Filed lune 24, 196%, Set. No. 35,531 4 Claims. (Cl. 117-212) This invention relates to methods of selective plating and particularly the selective plating of gold onto the alloy contacts of silicon semiconductor devices.

The invention will be described herein with reference to its particular application to silicon diodes having aluminum alloy contacts. As used herein, aluminum alloy is intended to embrace substantially pure aluminum as well as aluminum alloys formed during the alloying of rectifying PN junctions or used as the alloying material.

Due to the fact that aluminum and high aluminum alloys are characterized by the presence of a surface oxide layer, the attachment of lead wires to the aluminum alloy contacts of silicon diodes has been a bothersome problem because welding and soldering are rendered difiicult by the presence of the oxide layer. The present invention contemplates a method of selectively gold plating the aluminum alloy dot to facilitate making electrical contact therewith.

Lt is therefore the fundamental object of the present invention to provide a selective plating method.

More specifically it is an object of the present invention to provide a method of selectively gold plating the alloy dots of silicon semiconductor devices to the exclusion of the silicon wafer.

Another object is to provide a method of gold plating aluminum and other metals characterized by a rapidlyformed, tenacious surface oxide layer.

Another object is the provision of a method of gold plating the aluminum alloy contacts of silicon semiconductor devices Without deleterious effects on the rectifying junctions.

These and other objects of the invention are accomplished by the method of selective gold plating according to the present invention, which comprism soaking the object to be plated in a bath consisting of a heated solution of an alkali metal hydroxide selected from the group consisting of sodium hydroxide and potassium hydroxide; rinsing the object by progressive dilution of the hydroxide solution with hot water, While the object remains immersed, until the bath consists of substantially clear water;

admixing with the resultant Water rinse bath immersing the object, a gold-amino complex gold plating solution; and adjusting the pH of the bath to a value in the range of about 7 to 8.

Further objects of the invention, its advantages, scope and the manner in which it may be practiced are set forth in greater detail in the following description and the subjoined claims, taken in conjunction with the annexed drawing in which the single FIGURE is a how sheet repre senting the various method steps.

The first step of the method contemplated by the invention is the soaking of the completed diode or other object in a bath consisting essentially of a 2 N solution of sodium :or potassium hydroxide. The temperature of the solution preferably is maintained at about 80 C. The soaking is continued for about 3 or 4 minutes or suflicient time to effect removal of the oxide layer from the surface of the aluminum alloy button or contact of the diode without detrimental effect on its rectifying junctions or on the surface properties of the silicon wafer.

At the conclusion of the soaking period, the alkaline metal hydroxide bath is progressively and continuously diluted with hot, preferably boiling, water without removing the diode. The dilution is continued until the bath is Patented July 30, 1963 2. substantially devoid of hydroxide, i.e., is substantially plain water. The temperature of the bath preferably is maintained at or above C. during this operation.

The plain water bath resulting from the progressive dilution accomplishes rinsing of the diode without the exposure to the atmosphere.

To the resulting clear water bath is added a suitable gold-amino complex plating solution. A sufiicient quantity of ammonium hydroxide is added periodically, as necessary, to adjust and maintain the pH of the solution at a value between 7 and 8 as the ammonium hydroxide evaporate-s. As in the previous steps the solution temperature is maintained at about 80 0., plus or minus 5 C. The quantity of the gold-amino plating solution is, of course, a function of the volume of the rinse bath and the concentration necessary for plating. Following is a speciiic example of particular solutions and process parameters for gold plating silicon diode assemblies according to the present invention:

The diodes were first soaked in 50 cc. of a 2 N sodium hydroxide solution (4 grams sodium hydroxide/50 cc. H O) at a temperature of about 80 C. After 4 minutes immersion, the diodes were rinsed Without exposure to the atmosphere by pnogressive dilution of the bath with boiling Water, at the conclusion of which the 50 cc. volume of the bath was substantially pure water.

To this quantity of water was added 5 cc. of a goldamino complex solution known as Atomex, commercially available from Baker Contact Division, Engelhard Industries, Inc. Heat was applied as necessary to maintain the solution temperature at 80 C.:5 C. After about 1 minute, 2 cc. of 28 percent ammonium hydroxide solution was added to the plating bath to maintain alkalini'ty corresponding to a pH of between 7 and 8. Due to evolution of ammonia vapor (NH an additional 3 cc. of the 28 percent ammonium hydroxide solution was added after 3 minutes to maintain the desired pH. Plating was continued for 3 minutes longer after which the diodes were removed, rinsed and dried.

The addition of the Atomex solution to the water is believed to produce :a plating solution having the following approximate composition per liter of water:

Gr. Potassium gold cyanide 5 Ammonium citrate 20 Ethylene diamine tetra acetic acid 25 This and equivalent plating solutions are disclosed in French Patent No. 1,188,386 and further description is deemed unnecessary.

As a result of this treatment the diodes exhibited a smooth, bright, adherent gold plate on the aluminum dots with no plating whatever in evidence on the silicon surfaces. There was no evidence of plating of, or damage to, the rectifying junctions nor of deleterious effect on the silicon wafer.

It should be noted that it is important to avoid extreme departures, e.g., il0 C. from the 80 C. temperature specified. Generally speaking a somewhat lower temperature is acceptable for the hydroxide bath but in order to hasten and/ or facilitate arriving at the temperature of 80 C. i5", which is more or less critical for the plating bath, the dilution of the hydroxide bath should be accomplished with boiling or nearly boiling water so that the resultant rinse bath is at a temperature near C. This permits addition of the gold plating agent and ammonium hydroxide at room temperature without lowering the bath temperature much, if any, below 80 C. In this way, the application of heat from an external source can be minimized or avoided entirely.

The various time limits set forth are not critical although the time in the plating bath if unduly prolonged results in undesired plating of the rectifying junction,

It will be appreciated that the essential characteristic of the method contemplated by the invention involves the removal of oxides from the aluminum or aluminum alloy surface and maintaining the surface oxide free until after deposition of the gold plate without detrimental effect on the diode.

While there has been described what at present is believed to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed and desired to be secured by United States Letters Patent is:

1. A method of selectively gold plating the alloy contacts of a silicon semiconductor device, comprising: soaking the device in a hot solution of an alkali metal hydroxide selected from the group consisting of sodium hydroxide and potassium hydroxide; progressively diluting said solution to substantially clear hot water while said device remains immersed therein; admixing with the resultant rinse water in which the device is immersed, a chemical gold plating agent to produce a plating bath containing potassium gold cyanide, ammonium citrate, and ethylene diamine tetra acetic acid; and adjusting the pH of the plating bath to a value in the range of about 7 to 8.

2. A method of selectively gold plating the aluminum alloy contacts of a silicon semiconductor device comprising: soaking the device in a bath consisting essentially of a hot solution of an alkali metal hydroxide selected from the group consisting of sodium and potassium hydroxide; progressively diluting said bath to substantially clear hot water while said device remains immersed therein; admixing With the resultant rinse Water bath, while the device remains immersed therein, a chemical gold plating agent to produce a plating bath containing potassium gold cyanide, ammonium citrate, and ethylene diamine tetra acetic acid; adjusting and maintaining the pH of said solution at a value between 7 and 8 by additions of ammonium hydroxide solution; and maintaining the temperature of the bath containing the gold plating agent within the temperature range 7585 C.

3. A method of selectively gold plating the aluminum alloy contacts of a silicon semiconductor device, comprising: soaking the device in a bath consisting essentially of a 2 N solution of an alkali metal hydroxide selected from the group consisting of sodium and potassium hydroxide for a period of about 3 to 4- minutes while said solution is maintained at a temperature in the range of about to 90 C.; progressively diluting said bath to substantially clear hot Water while said device remains immersed therein and the temperature thereof is maintained in said range; admixing with the resultant rinse water bath, while the device remains immersed therein, a gold-amino complex solution to produce a plating bath containing potassium gold cyanide, ammonium citrate and ethylene diamine tetra acetic acid; adjusting the pH of the resultant plating bath to a value of between 7 and 8 by addition of ammonium hydroxide solution; maintaining said value of pH by additions of ammonium hydroxide solution as necessary; and maintaining the temperature of the plating bath Within the range i5 C. until completion of the plating.

4. A method of selectively gold plating the aluminum alloy contacts of a silicon semiconductor device, comprising: soaking the device in a bath consisting essentially of a hot 2 N solution of an alkali metal hydroxide selected from the group consisting of sodium and potassium hy droxide for a period of about 3 to 4 minutes; progressively diluting said bath to substantially clear boiling water while said device remains immersed therein; admixing with the resultant rinse Water bath, while the device remains immersed therein, a gold-amino plating solution to form a plating bath containing potassium gold cyanide, ammonium citrate and ethylene diamine tetra acetic acid; after about one minute adding to the plating bath 0.4 part by volume of ammonium hydroxide and, after an additional minute adding 0.6 part by volume of ammonium hydroxide; after an additional three minutes, rcmoving the device from the plating bath; and maintaining the temperature of the plating bath Within the range of about 75 to C. until completion of the plating.

References {Jilted in the file of this patent UNITED STATES PATENTS 2,733,167 Stoo-key Ian. 31, 1956 2,872,346 Miller Feb. 3, 1959 FOREIGN PATENTS 1,044,286 Germany Nov. 20, 1958 

1. A METHOD OF SELECTIVELY GOLD PLATING THE ALLOY CONTACTS OF A SILICON SEMICONDUTCTOR DEVICE, COMPRISING: SOAKING THE DEVICE IN A HOT SOLUTION OF AN ALKALI METAL HYDROXIDE SELECTED FROM THE GROUP CONSISTING OF SODIUM HYDROXIDE AND POTASSIUM HYDROXIDE; PROGRESSIVELY DILUTING SAID SOLUTION TO SUBSTANTIALLY CLEAR HOT WATER WHILE SAID DEVICE REMAINS IMMERSED THEREIN; ADMIXING WITH THE RESULTANT RINSE WATER IN WHICH THE DEVICE IS IMMERSED, A CHEMICAL GOLD PLATING AGENT TO PRODUCE A PLATING BATH CONTAINING POTASSIUM GOLD CYANIDE, AMMONIUM CITRATE, AND ETHYLENE DIAMINE TETRA ACETRIC ACID, AND ADJUSTING THE PH OF THE PLATING BATH TO A VALUE IN THE RANGE OF ABOUT 7 TO
 8. 